We propose to use loss of heterozygosity studies and DNA sequencing to identify a gene at 13q14 that is involved in the pathogenesis of B-CLL. We have constructed and characterized a YAC contig spanning a region of 3 cM between the RB gene and the D13S31 locus. We have also screened 60 paired normal/tumor B CLL cases for allelic loss on chromosome 13 with 9 microsatellite markers in the region. This analysis has allowed us to narrow the smallest region of loss to a segment of 550 kb located between the 206XF12 and D13S25 markers. We have completed construction of a cosmid and BAC contig of the region and are carrying out exon trapping and cDNA selection experiments in order to identify candidate genes. We have also taken advantage of our outstanding automated DNA sequencing facility to sequence the entire 500 kb region. We have completed sequencing a 792 kb region that includes the minimal region of loss and are currently screening identified ESTs and ORFs in order to aid in the identification of the gene that is mutated and/or deleted in B-CLL.